A typical input clamping structure 100 for a class AB power amplifier is shown in FIG. 1. Circuit 100 includes a reference voltage generator including a first leg comprising a first bias current I1 (typically between 70 and 100 microamps) coupled to node 104, serially coupled diodes D1, D2, D3, and D4, and resistor R1 (typically about 3K ohms). A second leg of the reference voltage generator comprises a second bias current I2 (typically between 70 and 100 microamps), a transistor Q1 having a drain for receiving the I2 bias current at node 102, a base coupled to node 104, and an emitter coupled to resistor R2 (typically about 30K ohms). The voltage at node 104 is about 3.3 volts. The two bias currents are also coupled to VDD, which is typically set at about 14.4 volts for a power amplifier. The clamping circuit also includes four NPN Darlington transistors Q2/Q3, Q4/Q5, Q6/Q7 for generating clamped input voltages PLAY1, PLAY2, PLAY4. The PLAY1 node is coupled through resistors R3 and R4 to the SVR node. The values of resistors R3 through R10 is about 25K ohms. The SVR node stands for “Supply Voltage Rejection” and is an internal reference node set to about VDD/4. The serially coupled resistors are shown for better matching and routing in layout design of an integrated circuit. Similarly, the PLAY2 node is coupled through resistors R5 and R6 to the SVR node, and the PLAY4 node is coupled through resistors R7 and R8 to the SVR node. The ACGND node is coupled through resistors R9 and R10 to the SVR node. Note that the value of resistors R9 and R10 is one fourth the value of the PLAY node resistors. ACGND is the common node corresponding to nodes PLAY1 to PLAY 4, as will be seen and described in further detail below, and so these resistors must be four resistors in a parallel connection.
In FIG. 2 PLAYx and CMP_ACGNDx are the direct inputs to the amplifier input stage. In the layout of the integrated circuit the star-point (the metal traces to the destination must be coming from the same point although each of the metal traces are at the same potential) is from ACGND in FIG. 2, and Rshort is the routing metal resistance. Resistor R1 is about 600 ohms and R1/4 is about 600 ohms, divided by four. Note that the resistors shown in FIG. 2 are different than those shown in FIG. 1. In FIG. 2, there are four ACGND nodes that correspond to four input nodes. The four ACGND nodes (CMP_ACGND1, CMP_ACGND2, CMP_ACGND3, and CMP_ACGND4) are coupled to the same point (ACGND) and the four inputs (PLAY1, PLAY2, PLAY3, and PLAY4) are independent. The capacitors shown in FIG. 2 represent discrete capacitors that are used in conjunction with the class AB amplifier integrated circuit.
The integrated circuit class AB amplifier 300 is shown in FIG. 3, including amplifier pair 302, amplifier pair 304, amplifier pair 306, amplifier pair 308, and clamp circuitry 310. The input and ground connections are all as previously described. In FIG. 3, the Rf resistors are typically 8K ohms, and the Re resistors are typically 400 ohms. The class AB amplifier 300 is shown in a typical audio application for driving loudspeakers.
While the amplifier 300 shown in FIG. 3 achieves a good level of performance, it is limited by the unbalanced ground connection. What is desired is a class AB amplifier that has the highest level of audio performance and least amount of distortion with a completely separate and balanced ground connection.